The basement membrane is a ubiquitous, specialized type of extracellular matrix separating organ parenchymal cells from interstitial collagenous stroma. Interaction of cells with this matrix is an important aspect of both normal and neoplastic cellular processes. Normal cells appear to require an extracellular matrix for survival, proliferation and differentiation, while migratory cells, both normal and neoplastic, must traverse the basement membrane in moving from one tissue to another. In particular, metastatic cancer cells arising in squamous or glandular epithelium must traverse the basement membrane to enter the circulatory and lymphatic systems (intravasation); the circulating neoplastic cells are typically arrested in the capillary beds of an organ, invade the blood vessel walls, and penetrate the basement membrane to extravascular tissue (extravasation), where a secondary neoplasm is then established. The mechanisms of cellular interaction with the basement membrane are thus of great interest.
The interaction of cells with extracellular matrices is dependent upon the ability of the cells to attach themselves to the matrix; it is known that this attachment may be mediated by specific glycoproteins which typically bind cells to discrete collagen types present in the matrix. Fibronectin-mediated attachment of fibroblasts, myoblasts, and smooth muscle cells to interstitial type I and type III collagen, and chondronectin-mediated attachment of chondrocytes to type II cartilage collagen, are exemplary.
It has been found that the attachment of both normal and neoplastic cells to the basement membranes is similarly mediated. The primary constituents of the basement membrane are type IV collagen, glycoproteins and proteoglycans. The glycoprotein laminin mediates the attachment of both epithelial and neoplastic cells to the basement membrane, binding the cells to type IV collagen. Since, as previously noted, metastasizing tumor cells must traverse basement membranes at multiple stages in the metastatic process, and since the first step in this process is tumor cell attachment to the basement membrane, the elucidation of this mechanism and the corollary characterization of specific attachment factors which promote or inhibit tumor cell attachment to this membrane has important implications for cancer diagnosis and management.
A number of studies show that TSP can bind to multiple cell surface receptors on the same cell or bind to different receptors on different cells. For example, platelets can bind TSP through GPII.sub.b -IIIa, GPI.sub.a -II.sub.a (Karczewski et al., J. Biol. Chem. (1989) 264:21332-21326 and Tuszynski et al., J. Clin. Invest. (1991) 87:1387-1394), and the vitronectin-receptor (Tuszynski et al., Exp. Cell Res. (1989) 182:481). Smooth muscle cells, endothelial cells, U937 monocyte-like cells, and melanoma cells can bind TSP through a vitronectin-like receptor. Squamous cell carcinoma bind TSP through a Mr 80,000/105,000 that is not an integrin or GPIV (Yabkowitz et al., Cancer Res. (1991) SI:3648-3656).
Receptors for other extracellular matrix proteins have been isolated. Liotta et al. (U.S. Pat. No. 4,565,789) describe the isolation of a laminin receptor. Mecham et al. (J. Biol. Chem. (1989) 264:16652-7) describe an elastin receptor which exhibits structural and functional similarity to the 67 kd laminin receptor. CD36 has been implicated as binding the Cys Ser Val Thr Cys Gly (SEQ ID NO: 1) sequence of thrombospondin (Asch et al., Biochem. Biophys. Res. Comm. (1992) 182:1208-1217). However, CD36 is an 88 kd protein. The Cys Ser Val Thr Cys Gly (SEQ ID NO: 1)-specific receptor of the present invention is different from these previously isolated extracellular matrix protein receptors.